The present invention relates to centrifuges of the decanting type.
Decanting type centrifuges employ a bowl which rotates about a horizontal or vertical axis and contains a helical scroll conveyor to separate a slurry fed thereto into its constituent solids and liquid. The helical conveyor rotates at a slightly different speed within the bowl to scroll the heavier solids to discharge ports at the smaller diameter end of the bowl. The separated liquid flows in the opposite direction and is discharged from ports at the opposite end of the bowl. The decanter can be of two principle types, either solid bowl or screen bowl. In the latter, the solids are scrolled by the conveyor over an additional perforated screen section of the bowl prior to discharge.
Existing decanter centrifuges of both the solid and screen bowl types operate when fed with a slurry containing solids with a higher specific gravity than the liquid constituent of the slurry either to:--
(a) separate the solid particles from the liquid, or to
(b) classify the solids, that is to divide the solids so that particles above a certain size are discharged as solids and particles below that size are discharged with the liquid.
For both separation and classification, the rotation of the decanter applies centrifugal force to the slurry to promote rapid settling of the higher specific gravity for scrolling and discharge. In the description that follows, the words "separate" and "separation", when applied to solids and liquids, includes "classify" and "classification".
FIG. 1 of the accompanying drawings shows, in part section, a conventional state-of-the-art solid bowl decanter designed to rotate about axis XX and to separate slurry fed via feed pipe (1) and feed ports (2) into the bowl (3), which includes a cylindrical section (3A) joined to a section shaped as a frustrum of a cone (3B)--herein referred to as the conical bowl section. The slurry, subjected to centrifugal force, fills the bowl to the inner surface (4) determined by the radial position of the liquid outlet ports (5). A conveyor hub (6) coaxially mounted within the bowl (3) and supported on bearings (7), carries scrolling flights (8) wound in a helix and attached to the hub (6). The plane of the scrolling flights tilts forward to subtend an angle (a), typically 0.degree.-4.degree. to the generator of the cylindrical (3A) or the conical (3B) sections of the bowl (3). A gearbox (not shown) drives the conveyor (6) in the same rotation but at a speed slightly different from the bowl (3) such that the flights scroll towards the solids discharge end (9) of the decanter. Under centrifugal force, the solids (10) settle rapidly on the bowl wall and are scrolled by the conveyor flights (8) and discharged from the solids outlet (11) whilst the liquid, after primary separation, flows from the outlet (5).
The centrifugal force produced by rotation results in compressive forces on the solids. For the solids in the conical bowl section (3B) and scrolled clear of the liquid surface (4), the compressive forces are zero at the minimum solids radii and increase linearly to a maximum value at the inner wall of the conical bowl section. The solids immersed in the liquid are subjected to a compressive force applied by the liquid "head" which again is zero at the liquid surface (4) and a maximum at the inner bowl wall. In the description that follows the words "compression" and "compressive forces" apply only to forces applied mechanically to the solids by the decanter components and do not include the compressive forces induced directly by rotation.
An object of the present invention is to improve the design of conventional decanting centrifuges so that, in addition to separating the slurries of solids and liquid as described above (the primary separation), the part-dried solids are also subjected to applied compressive forces during scrolling to remove additional liquid before being discharged from the bowl (the secondary and tertiary separations).
A further object is to collect the liquid extracted by the primary and subsequent separations in individual streams for further processing.